The prion diseases are a family of transmissible neurodegenerative disorders that affect humans and animals. A large body of evidence argues that a post-translational, non-covalent modification of the prion protein (PrP) is the fundamental event in the mechanism underlying these diseases. The normal cellular isoform (PrPC) is misfolded to the beta-sheet rich pathogenic isoform (PrPSc). Once formed, PrPSc appears to act as a conformational template to convert additional PrPC to PrPSc. Considerable evidence supports sequence homology within the central region of PrP as a necessary feature in the association of PrPSc with PrPC as a prelude to conversion and propagation of PrPSc, but the exact segment(s) involved and the sequence determinants for this conversion are unknown. Studies targeted at understanding the site(s) of association of PrPC and PrPSc will no doubt define ways to inhibit their interaction and provide treatment for these currently untreatable diseases. The proposed studies are designed, therefore, to define the molecular determinants within PrP, and primarily within the putative priori domain, that are required for efficient self-association and conformational transfer. We will primarily utilize transgenic mice that express polymorphic PrP genes to act as hosts for a variety of sporadic and genetic human prion diseases to determine if and where homologous regions and residues are required for efficient transmission of prion strain. In addition a novel yeast-based model of prion disease will be extensively utilized to study the effect of specific substitutions or deletions at potentially critical association sites of PrP on the development of PrPSC-like protein. This powerful model is not only capable of generating prpSC-Iike protein, but can support the de novo generation of at least two strains of PrPSc so, and demonstrate conformational transference of PrPSc to PrPC. The information gained from these studies will then be applied to the development of novel peptide inhibitors that are designed to bind to critical sites within the defined "prion domain" and halt additional binding of PrP. With the continued threat of bovine spongiform encephalopathy in Europe, and the current spread of chronic wasting disease of deer and elk in the U.S., these studies are urgently needed.